Offset multiaxial or polyaxial screw, system and assembly

ABSTRACT

A capless multiaxial screw fixation assembly, including: a screw having a threaded portion and a screw head portion positioned at one end thereof; and, a retainer having a first end, a second end opposite the first end, and a bore for receiving the screw threaded portion so that the screw head portion is seated therein. At least one of the screw and the retainer is configured such that the retainer is able to pivot about the screw head portion in a non-symmetrical manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a screw having an offset headand to a multiaxial or polyaxial screw for use with a bone fixationsystem and, in particular, to a multiaxial or polyaxial screw whichpermits a greater range of pivotability in at least one direction.

2. Description of the Related Art

As is generally known in the art, spinal abnormalities may becorrectable using a pair of posterior spinal fixation rods attached tothe vertebrae using pedicle screws and the like. In order to provideincreased stability and rigidity, especially to resist twisting or thelike, the pair of elongated rods often include cross connecting devices.The cross connecting devices typically traverse the spinal column andcouple to each of the elongated rods. In other words, the crossconnecting devices are perpendicular or substantially perpendicular tothe spinal column.

In addition, bone screws with a polyaxial head are commonly used inspine surgery today. They are used chiefly in the lumbar spine andscrewed into bone (pedicle) posteriorly. The head of the screw isattached to the shaft of the screw. The head of the screw is machinedinto a ball, and the head may be provided with a receiver or socket intowhich the ball fits. One typical prior art system further contains areceiver for receiving a separate rod. The rod is fastened to the screwhead receiver via a threaded cap. The rod is then fastened to screwsplaced in adjacent vertebrae thus providing stabilization. The polyaxialhead allows the rod to be placed in a variety of angles with respect tothe screw allowing conformance to local anatomy.

Exemplary bone screws are disclosed in the following patents: U.S. Pat.No. 5,466,237 to Bird et al.; U.S. Pat. No. 4,946,458 to Harms; U.S.Pat. No. 5,207,678 to Harms et al.; U.S. Pat. No. 5,474,555 to Puno etal.; and U.S. Pat. No. 6,869,433 to Glascott. It will be appreciatedfrom the prior art, however, that multiaxial screws involve the abilityto pivot symmetrically or the same amount in each direction. In someinstances, it is desired for the retainer to be moved or pivoted aboutthe screw head more in a certain direction than in another, wherebygreater maneuverability of the rod attached to the retainer may beaccommodated. It was difficult to get a “favorable angle” of theretainer relative to the screw head.

FIGS. 1-3B show a prior art system wherein a retainer having an angledor canted surface that permits movement between a predetermined angle A(FIG. 2) and a different smaller angle B (FIG. 3A) when the retainer ismoved between a first direction C and a second direction D,respectively.

Thus, it would be desirable for a multiaxial screw assembly to bedeveloped for use with a spinal fixation system which permitsnon-symmetrical pivoting and that is capable of use with a variety ofretainers, including those with non-canted surfaces. It would also bedesirable for a multiaxial screw assembly to be developed in whichsimple modification of existing components enables the desired pivotingaction.

SUMMARY OF THE INVENTION

It is therefore, an object of the invention to provide a polyaxial screwhaving a head that provides a greater range of mobility.

In one aspect, one embodiment comprises a multiaxial screw fixationassembly, comprising a screw having a threaded portion and a screw headportion positioned at one end thereof, and a retainer having a firstend, a second end opposite said first end, and a bore for receiving saidthreaded portion so that said screw head portion may be seated therein,wherein said screw is adapted such that said retainer is able to pivotabout said screw head portion in a non-symmetrical manner.

In another aspect, another embodiment comprises a multiaxial screwfixation assembly, comprising a screw, further comprising a threadedportion having a centerline axis extending therethrough, and a headportion positioned at one end of said threaded portion, said headportion having a centerline axis extending therethrough, wherein saidscrew head portion is not aligned with said screw threaded portion, anda retainer having a first end, a second end opposite said first end, acenterline axis extending therethrough, and a bore for receiving saidscrew threaded portion so that said screw head portion is seated thereinaxis, wherein said retainer is able to pivot about said screw headportion in a non-symmetrical manner.

In still another aspect, another embodiment comprises a retainer for usein a spinal fixation procedure, said retainer comprising a retainer bodyhaving a first end for receiving a rod, said retainer also comprising abore for receiving a shank of a screw having a screw head, said retainerbody having a second end adapted to permit said retainer to pivot ormove in a first direction a first predetermined angle and furtheradapted to permit the retainer to pivot or move in a second direction asecond predetermined angle, wherein said second predetermined angle isless than said first predetermined angle.

In yet another aspect, another embodiment comprises a polyaxial screwfor use during a spinal procedure, said polyaxial screw comprising athreaded shank having a shank axis, and a head integral with saidthreaded shank, said head being adapted to permit a retainer to pivot ormove about a head axis, said head axis being offset from said shankaxis.

These and other objects and advantages of the invention will be apparentfrom the following description, the accompanying drawings and theappended claims.

BRIEF DESCRIPTION OF THE OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a front elevation view of a prior art retainer having acentered bottom edge;

FIG. 2 is a front elevation view of a prior art retainer, wherein theretainer associated therewith is shown in a first pivoted position;

FIG. 3A is a front elevation view of the prior art retainer assemblydepicted in FIGS. 1 and 2, wherein the retainer associated therewith isshown in a second pivoted position;

FIG. 3B is a top view of the prior art retainer taken along the line3B-3B in FIG. 1;

FIG. 4 is an exploded perspective view of the polyaxial screw assemblyhaving an offset head, wherein a compression member and a rod forengagement with the compression member are also shown;

FIG. 5 is an exploded side view of the polyaxial screw assembly depictedin FIG. 4;

FIG. 6 is a top view of the polyaxial screw assembly depicted in FIGS. 4and 5 taken along line 6-6 in FIG. 5;

FIG. 7 is a top sectional view of the polyaxial screw assembly depictedin FIGS. 4-6 taken along line 7-7 in FIG. 5;

FIG. 8 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIGS. 4 and 5, wherein a tool is shown asinterfacing with a head portion thereof to position it within theretainer;

FIG. 9 is a top sectional view of the polyaxial screw assembly with thehead portion thereof positioned within the retainer, as depicted in FIG.8, taken along line 9-9 of FIG. 8;

FIG. 10 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIG. 8, wherein the compression member and rodengaged therewith are included and the retainer has been pivoted to amaximum degree in a first direction;

FIGS. 11A and 11B are enlarged, partial sectional views of the polyaxialscrew assembly depicted in FIG. 8, wherein the compression member androd in unlocked and locked positions and the retainer has been pivotedto a maximum degree in a second direction;

FIG. 12 is an exploded perspective view of a polyaxial screw assemblyhaving a first alternative configuration, wherein a compression memberand a rod for engagement with the compression member are also shown;

FIG. 13 is an exploded side view of the polyaxial screw assemblydepicted in FIG. 12;

FIG. 14 is a top view of the polyaxial screw assembly depicted in FIGS.12 and 13 taken along line 14-14 in FIG. 13,

FIG. 15 is a top sectional view of the polyaxial screw assembly depictedin FIGS. 12 and 13 taken along line 15-15 in FIG. 13;

FIG. 16 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIGS. 12 and 13, wherein a tool is shown asinterfacing with a head portion thereof to position it within theretainer;

FIG. 17 is a top section view of the polyaxial screw assembly with thehead portion thereof positioned within the retainer, as depicted in FIG.26, taken along line 17-17 of FIG. 16;

FIG. 18 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIG. 16, wherein the compression member and rodengaged therewith are included and the retainer has been pivoted to amaximum degree in a first direction;

FIGS. 19A and 19B are enlarged, partial sectional views of the polyaxialscrew assembly depicted in FIG. 16, wherein the compression member androd in unlocked and locked positions and the retainer has been pivotedto a maximum degree in a second direction;

FIG. 20 is an exploded perspective view of a polyaxial screw assemblyhaving a second alternative configuration, wherein a compression memberand a rod for engagement with the compression member are also shown;

FIG. 21 is an exploded side view of the polyaxial screw assemblydepicted in FIG. 20;

FIG. 22 is a top view of the polyaxial screw assembly depicted in FIGS.20 and 21 taken along line 22-22 in FIG. 21,

FIG. 23 is a top sectional view of the polyaxial screw assembly depictedin FIGS. 20 and 21 taken along line 23-23 in FIG. 21;

FIG. 24 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIGS. 20 and 21, wherein a tool is shown asinterfacing with a head portion thereof to position it within theretainer;

FIG. 25 is a top section view of the polyaxial screw assembly with thehead portion thereof positioned within the retainer, as depicted in FIG.24, taken along line 25-25 of FIG. 24;

FIG. 26 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIG. 24, wherein the compression member and rodengaged therewith are included and the retainer has been pivoted to amaximum degree in a first direction;

FIGS. 27A and 27B are enlarged, partial sectional views of the polyaxialscrew assembly depicted in FIG. 24, wherein the compression member androd in unlocked and locked positions;

FIG. 28 is an exploded perspective view of a polyaxial screw assemblyhaving a third alternative configuration, wherein a compression memberand a rod for engagement with the compression member are also shown;

FIG. 29 is an exploded side view of the polyaxial screw assemblydepicted in FIG. 28;

FIG. 30 is a top view of the polyaxial screw assembly depicted in FIGS.28 and 29 taken along line 30-30 in FIG. 29,

FIG. 31 is a top sectional view of the polyaxial screw assembly depictedin FIGS. 28 and 29 taken along line 31-31 in FIG. 29;

FIG. 32 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIGS. 28 and 29, wherein a tool is shown asinterfacing with a head portion thereof to position it within theretainer;

FIG. 33 is a top sectional view of the polyaxial screw assembly with thehead portion thereof positioned within the retainer, as depicted in FIG.32, taken along line 33-33 of FIG. 32;

FIG. 34 is an enlarged, partial sectional view of the polyaxial screwassembly depicted in FIG. 32, wherein the compression member and rodengaged therewith are included and the retainer has been pivoted to amaximum degree in a first direction;

FIGS. 35A and 35B are enlarged, partial sectional view of the polyaxialscrew assembly depicted in FIG. 32, wherein the compression member androd engaged therewith in unlocked and locked positions;

FIG. 36 is an exploded view of another embodiment of the inventionillustrating the polyaxial screw assembly used in combination with aretainer that utilizes a cap, such as a thread cap;

FIG. 37 is a sectional exploded view of the embodiment shown in FIG. 36;

FIG. 38 is a view of another embodiment of the invention wherein a screwhead has a common or coaxial axis with the shank;

FIG. 39 is a sectional view of the embodiment shown in FIG. 38;

FIG. 40 is a sectional view taken along the line 40-40 in FIG. 39;

FIG. 41 is a fragmentary sectional view of the embodiment shown in FIG.39 illustrating the use of a tool that becomes coaxial with the axes andshank;

FIG. 42 is a sectional view taken along the line 42-42 in FIG. 41;

FIG. 43 is a fragmentary sectional view of the embodiment shown in FIG.38 illustrating the pivotal movement of the retainer relative to thehead, with the retainer being in an unlocked position;

FIG. 44 is another fragmentary sectional view of the embodiment shown inFIG. 38 illustrating the pivotal movement of the head in a directionthat is different from the direction shown in FIG. 43, but illustratinga greater degree of pivoting; and

FIG. 45 is a sectional view of an intermediate portion showing itseccentricity and non-symmetry about an axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings in detail, wherein identical numeralsindicate the same elements throughout the figures, FIGS. 4-35B depictvarious embodiments of a multi-axial screw and spinal fixation assembly10. Spinal fixation assembly 10 includes a screw 12 having a threadedportion 14 and a screw head 16 that preferably has a rounded, arcuate,spherical or curved profile. Spinal fixation assembly 10 furtherincludes a retainer 18, which preferably has a generally cylindricalconfiguration. In these illustrative embodiments, the retainer 18 iscapless, but the retainer 18 could be non-capless, such as isillustrated in FIGS. 36 and 37 below. Although not shown, it will beappreciated that retainer 18 includes an aperture or bore 15therethrough along a centerline axis 20 (FIG. 4). Accordingly, the bore15 that receives a threaded portion 14 of the screw 12 until screw head16 thereof is received adjacent a first end 18 a of retainer 18. It willbe understood that screw head 16 may be positioned within retainer 18 inany known manner so that multiaxial or polyaxial and relative movementbetween retainer 18 and screw 12 is permitted. In this way, a user, suchas a surgeon or physician, is able to change the polyaxial position ofretainer 18 relative to screw 12 in order to adjust an angular positionof the retainer or receiving channel 19 and an elongated member or rod,such as rod 28 (FIG. 6), which is received and retained in the retainer18 at a first end 18 a.

Contrary to similar assemblies in the prior art shown in FIGS. 1-3B,screw 12 is configured so that the retainer 18 is able to pivot in anon-symmetrical manner about screw head 16. More specifically, it willbe noted from the embodiment shown in FIGS. 10, 11A-11B, 18, 19A-19B,26, 27A-27B, 34 and 35A-35B, that a centerline axis 16 a of screw head16 is offset from a centerline axis 14 a of the threaded portion 14. Thescrew head 16 enables the retainer 18 to pivot by varying degrees indifferent directions. In this way, implementation of spinal fixationassembly 10 will permit a greater degree of movement of the retainer 18toward the threaded portion 14, which may be desirable for a givenapplication or case.

In the illustration, the retainer 18 which preferably has a generallycylindrical configuration with helical rod-receiving channels 21 and 23(FIG. 4) similar to that described in detail in U.S. Patent ApplicationPublication 2007/0043357 A1 to Kirschman U.S. patent application Ser.No. 11/193,523 filed Jul. 29, 2005; U.S. patent application Ser. No.11/610,698 filed Dec. 14, 2006; and U.S. patent application Ser. No.11/762,911 filed Jun. 14, 2007, which are also owned by the assignee ofthe present application and all of which are incorporated by referenceand made a part hereof.

It will be appreciated that retainer 18 preferably includes a receivingchannel 19 in communication with the helical rod-receiving channels 21and 23 for engaging an elongated rod or member 28. In one embodiment,the rod 28 may engage directly against the screw head 16.

The spinal fixation assembly 10 may further include a compression member24 that is received in the bore 15. The compression member 24 comprisesa receiving channel 26 which is utilized to receive engage rod 28 andwhich engages the screw head 16.

As mentioned earlier, the screw 12 may be used with a non-caplessretainer 18, such as retainer 18′ in FIGS. 36 and 37, which utilizes acap or other means for retaining rod 28 in the retainer.

While a centerline axis 14 a through threaded portion 14 of screw 12remains generally aligned with a centerline axis 16 a through retainer18, it will be appreciated that a centerline axis 16 a through screwhead 16, however, is generally offset from and not coaxial withcenterline axes 14 a and 16 a. As such, screw head 16 may either berotated so that centerline axis thereof is at a specified angle relativeto centerline axes 14 a and 16 a or shifted in parallel to suchcenterline axes 14 a and 16 a. In either case, it will be understoodthat screw head 16 is not configured or oriented so as to be symmetricalwith respect to or about the centerline axes 14 a and 16 a.

Screw head 16 preferably includes at least one female opening, slot orgroove 30 (FIG. 4) so that a corresponding tool 34 is able to interactwith screw 12 in order to threadingly screw the screw 12 into spinalbone. The opening 30 to be formed therein for receiving thecorresponding tool 34 which enables screw 12 to be threaded into and outspinal bone. As best seen in FIGS. 6 and 9, the partial opening isconfigured as walls 31 a, 31 b, 31 c and 31 d that intersect at point 32and that define recessed areas or slots 30 formed in screw head 16(i.e., as for a Phillips screw). More specifically, the walls 31 a, 31b, 31 c and 31 d define opening 30 in screw head 16. Note that theintersecting point 32 is aligned with centerline axis 14 a of threadedportion 14 or a distance 35 (FIG. 7) from centerline axis 16 a. In thisway, the tool 34 (FIG. 8) is able to access the screw head 16 and slots30 and pressure applied thereby is directed along centerline axis 14 a.

When screw head 16 is seated properly within retainer 18, as viewed forexample in FIGS. 8, 10 and 11, it will be appreciated that theintentional misalignment of screw head 16 and threaded portion 14creates the ability for retainer 18 (and compression member 24) to pivotnon-symmetrically. For example, FIG. 10 depicts the pivoting of retainer18 about screw head 16 in a first direction E as an angle 38 with arange of, for example, approximately 0°-55°. Similarly, FIG. 11A showsthe pivoting of retainer 18 about screw head 16 in a second direction F(opposite the first direction), where an angle 40 has a range of, forexample, approximately 0°-55°. Clearly, the maximum amount of movementor pivoting is greater in the first direction E than in the seconddirection F. This permits a desirable flexibility for the adjustment ofrod 28 when secured as part of a spinal fixation assembly.

FIG. 11B shows the rod 28 in a loaded and locked position after theretainer 18 has been pivoted and rotated to the locked position.

It will be understood that the partial opening or partial shape of thescrew head 16 and the corresponding tool or driver utilized to interfacetherewith may take any desirable form. Besides the exemplary slots 30and tool 34 shown in FIGS. 6, 8 and 9, other possible configurations ofthe screw head 16 include, but are not limited to a shape adapted todefine a hexagonal opening 42 (FIGS. 14 and 17), a “Y” shape 44 (FIGS.22 and 25), and an opening 46 defined by a plurality of side walls(FIGS. 30 and 33). Of course, corresponding tools 48 (FIG. 16), 50 (FIG.24) and 52 (FIG. 32) would be utilized therewith, respectively. It willbe appreciated that the respective tool may have a linear configuration(as evidenced by tools 34 and 48) or a non-linear configuration (as seenfor tools 50 and 52).

Moreover, the embodiments of FIGS. 4-35B illustrate use of the screw 12with the compression module 17, but it should be understood that thescrew 12 may be used with retainers that do not utilize a compressionmodule 17. It should be understood that while the embodimentsillustrated in FIGS. 4-35B illustrate the use of the screw 12 with acapless retainer and compression module 17, the screw 12 may be usedwith non-capless systems. For example, FIGS. 36 and 37 illustrate theuse of the screw 12 with a non-capless retainer 18′. In this embodiment,retainer 18′ has a threaded interior surface for receiving a threadedcap 39. Of course, the screw 12 may also be used with other types ofretainers that utilize other means for retaining the rod in thereceiver, and such receivers and securing mechanisms are known to thoseskilled in the art.

Referring now to FIGS. 38-44, another embodiment is shown. In thisembodiment, those parts that are the same or similar to the part shownin FIGS. 4-35B are identified with the same part number.

As shown in FIGS. 38 and 39, notice that this embodiment utilizes ascrew 102 having a threaded shank 104 and a head 106. The head 106comprises a female aperture 108 for receiving a tool for screwing thescrew 102. In the illustration being described, and as illustrated inFIG. 41, the tool 110 may be a linear tool, without an offset of thetype illustrated in FIG. 24, for example. Notice that in thisembodiment, the receiver 18 comprises an axis 112 that is coaxial withthe axis 104 a of the shank 104. Notice also that the head 106 comprisesan axis 106 a that is coaxial with both the axis 104 a of shank 104 andthe axis 112 as illustrated in FIG. 39. This is advantageous in thatwhen the screw 112 is rotatably driven, it may be driven along a commonaxis which facilitates mounting or screwing the screw 102 into bone.

As best illustrated in FIGS. 38 and 39, notice that the head 106 isintegrally secured to the shank 104 by an intermediate or neck portion113. As shown in FIGS. 41-43 and 44, notice that the neck portion 113comprises a first surface or area 112 a having a relatively large radiusand a second surface or area 112 b that has a relative small radius asshown. The larger and small radiuses of the surfaces 112 a and 112 bpermit the retainer 18 to be pivoted a plurality of different angles ordegrees. For example, FIG. 43 illustrates the retainer 18 pivoting tothe left (as viewed in FIG. 43) a predetermined angle 114 as shown. Incontrast, notice in FIG. 44, the relatively small radius surface 112 bdefines an area or intermediate portion 113 that permits the retainer 18to pivot toward the right (as viewed in FIG. 44) a predetermined angle116 which is greater than the predetermined angle 114 as in at least onearea of the intermediate portion 113.

To enable the polyaxial movement, notice in FIG. 41 that theintermediate portion 113 is neither centric nor symmetrical about theaxis 104 a. Thus, the intermediate portion 113 is eccentric about axis104 a. Notice in FIGS. 38-45 that the radius of curvature is notconstant about the axis 104 a and generally becomes larger as the radiusof curvature moves from the portion or area 112 b toward the area 112 a,with the area 112 a having the largest radius of curvature. Thus, notonly is the intermediate portion 113 not symmetrical about the axis 104a, but the radius of curvature changes as well.

This embodiment enables the system of FIGS. 38-45 to enable the retainer18 to pivot a plurality of angles about the head 106, with the greatestdegree of pivot being realized when the retainer 18 pivots toward thesmallest radius of curvature 112 b and the smallest amount of pivotbeing achieved when the retainer 18 is pivoted toward the largest radiusof curvature 112 a. Thus, the embodiment of FIGS. 38-45 enables theretainer 18 to pivot a plurality of different angles about the head 106as shown.

Another advantage of the embodiment illustrated in FIGS. 38-44 is thatit enables the use of the tool 110 that is generally linear and that isnot offset as illustrated, for example, in the embodiment shown in FIG.24. This facilitates enabling the tool 110 to become coaxial with theaxes of the head 106 and shank 104, as illustrated in FIG. 41, whichfacilitates rotating the screw 104 and mounting in into bone.

FIG. 42 shows a cross-section of the illustration shown in FIG. 41illustrating the tool 110 being coaxial with the head 106 as shown.

Note also that an intersection of the screw head 16 to the threadedportion 14 may have a wall 14 b (FIGS. 5 and 8) that defines an area 14c that is generally concave and permits a bottom edge 18 a 1 (FIGS. 4, 5and 8) of retainer 18 to move inwardly toward centerline axis 14 a andinside an imaginary plane defined by the outside edge 14 d (FIG. 8) ofat least one thread on the threaded portion 14.

Having shown and described the preferred embodiment of the presentinvention, further adaptations of the capless multiaxial screw can beaccomplished by appropriate modifications by one of ordinary skill inthe art without departing from the scope of the invention.

While the system and method herein described constitute preferredembodiments of this invention, it is to be understood that the inventionis not limited to this precise system and method, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

1. A multiaxial screw fixation system, comprising: (a) a screw having athreaded portion and a screw head portion positioned at one end thereof;and (b) a retainer having a first end, a second end opposite said firstend, and a bore for receiving said threaded portion so that said screwhead portion may be seated therein; wherein said screw is adapted suchthat said retainer is able to pivot about said screw head portion in anon-symmetrical manner.
 2. The multiaxial screw fixation system of claim1, said screw comprises an offset screw head.
 3. The multiaxial screwfixation system of claim 2, wherein said offset screw head permits saidretainer to pivot with respect to said screw head portion in a firstdirection greater than in a second direction opposite of said firstdirection.
 4. The multiaxial screw fixation system of claim 1, whereinsaid retainer is a capless retainer.
 5. The multiaxial screw fixationsystem of claim 1, wherein said retainer comprises a cap for retaining arod in the retainer.
 6. The multiaxial screw fixation system of claim 1,wherein a centerline axis through said screw is co-linear with acenterline axis of said retainer.
 7. The multiaxial screw fixationsystem of claim 1, said threaded portion having a first centerline axisextending therethrough and said screw head portion having a secondcenterline axis extending therethrough, wherein said first centerlineaxis and said second centerline axis are not co-linear.
 8. Themultiaxial screw fixation system of claim 7, wherein said secondcenterline axis through said screw head portion is offset from saidfirst centerline axis of said threaded portion.
 9. The multiaxial screwfixation system of claim 7, wherein said second centerline axis of saidscrew head portion is oriented at an angle to said first centerline axisof said threaded portion.
 10. The multiaxial screw fixation system ofclaim 1, wherein said screw comprises an intermediate portion betweensaid screw head portion and said threaded portion; said intermediateportion being non-symmetrical about its axis.
 11. The multiaxial screwfixation system of claim 1, wherein said screw comprises an intermediateportion coupling said threaded portion to said screw head portion; saidintermediate portion comprising a radius of curvature that changes aboutthe axis of the threaded portion.
 12. The multiaxial screw fixationsystem of claim 11, wherein said screw head portion and said threadedportion comprise a common axis, said intermediate portion beingnon-symmetrical or non-concentric about the common axis.
 13. Themultiaxial screw fixation system of claim 7, wherein said screw headportion is not symmetrical with respect to said first centerline axis ofsaid threaded portion.
 14. The multiaxial screw fixation system of claim7, said screw head portion further comprising a partial opening formedtherein for receiving a corresponding tool.
 15. The multiaxial screwfixation system of claim 14, wherein a centerpoint of said partialopening is offset from said second centerline axis of said screw headportion.
 16. The multiaxial screw fixation system of claim 14, wherein acenterpoint of said partial opening is aligned with a first centerlineaxis of said threaded portion.
 17. The multiaxial screw fixation systemof claim 14, wherein said partial opening is a pair of intersectingslots.
 18. The multiaxial screw fixation system of claim 14, whereinsaid partial opening is at least one of a hexagonal opening or ay-shaped slot.
 19. The multiaxial screw fixation system of claim 1, saidretainer including a receiving channel for receiving an elongatedmember.
 20. The multiaxial screw fixation system of claim 1, furthercomprising a compression member for situating in said bore and engagingsaid screw head portion.
 21. A multiaxial screw fixation system,comprising: (a) a screw, further comprising: (1) a threaded portionhaving a centerline axis extending therethrough; and (2) a head portionpositioned at one end of said threaded portion, said head portion havinga centerline axis extending therethrough, wherein said head portion isnot aligned with said threaded portion; and (b) a retainer having afirst end, a second end opposite said first end, a centerline axisextending therethrough, and a bore for receiving said threaded portionso that said head portion may be seated therein; wherein said retaineris able to pivot an axis of said threaded portion in a non-symmetricalmanner.
 22. The multiaxial screw fixation system of claim 21, whereinsaid centerline axis of said threaded portion and said centerline axisof said head portion are not aligned.
 23. A polyaxial screw for useduring a surgical procedure, said polyaxial screw comprising: a threadedshank having a shank axis; and a head integral with said threaded shank,said head being adapted to permit a retainer to pivot or move about ahead axis, wherein said head axis being offset from said shank axis. 24.The polyaxial screw as recited in claim 23, wherein said head isgenerally spherical and comprises at least one recessed area adapted toreceive a tool for rotating said polyaxial screw.
 25. The polyaxialscrew as recited in claim 24, wherein said at least one recessed areacomprises a plurality of recessed areas.
 26. The polyaxial screw asrecited in claim 24, wherein said at least one recessed area comprises aplurality of recessed areas defines a y-shaped engaging area or aplurality of non-intersecting recessed areas.
 27. The polyaxial screw asrecited in claim 23, wherein an axis of said head is offset from saidshank axis.
 28. The polyaxial screw as recited in claim 23, wherein saidthreaded shank has a wall defining a recessed area to permit an end ofsaid retainer to pivot toward said shank axis and inside an imaginaryline defined by an outside edge of at least one thread of said threadedshank.
 29. A polyaxial screw for use during a surgical procedure, saidpolyaxial screw comprising a: a threaded shank having a shank axis; ahead having a head axis; and an intermediate portion integrally couplingsaid threaded shank to said head; said head and said threaded shankhaving a common axis and being centric or non-symmetrical about saidaxis.
 30. The polyaxial screw as recited in claim 29, wherein saidintermediate portion comprises a radius of curvature that changes aboutthe axis.